add timezone context in new ui

[bbovenzi]

Adding a context provider to handle what timezone to use in the new UI.

The order of operations is:=

1. local timezone of the browser
2. fall back to UTC

• we plan to add a GET /ui-metadata endpoint to act as the primary source of truth. that will be put in when the API is ready

Closes #14806

This is just for display purposes. Any changes by the user will be handled in #14807

• created a /providers directory and moved auth there too
• we were originally going to use /config for the default_timezone but then removed it in favor of /ui-metadata. But we're leaving the config hook and interfaces in there because it will be useful later.

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[ryanahamilton]

LGTM

[github-actions]

The PR most likely needs to run full matrix of tests because it modifies parts of the core of Airflow. However, committers might decide to merge it quickly and take the risk. If they don't merge it quickly - please rebase it to the latest master at your convenience, or amend the last commit of the PR, and push it with --force-with-lease.